The present invention relates to an ink jet printer; and, more particularly, the invention relates to a structure of a printing head and an ink circulating method.
An ink jet printer is known wherein ink droplets are ejected each in a small quantity from a very small ink ejecting portion and are deposited on a recording medium to form dots, allowing an image to be printed by a gathering of those dots. According to the configuration of such an ink jet printer, ink is conducted to the ink ejecting portion, in which a kinetic energy is imparted to the ink, thereby allowing an ink droplet to be ejected from the ink ejecting portion and deposited on a surface of the recording medium to form a dot. As one of the driving methods for imparting a kinetic energy to the ink, there is a known method wherein a piezoelectric element is bonded as a drive member to a part of a wall which constitutes an ink chamber, and a pulse voltage is applied to the piezoelectric element, causing the piezoelectric element to be deformed instantaneously, thereby deforming the above mentioned wall to decrease the volume of the ink chamber. Then, due to the resulting pressure, the ink is ejected from a small hole of 30 to 50 xcexcm in diameter. For example, in Japanese Patent Laid-Open No. Hei 8-58089 there is described an ink jet printer in which the above method is used.
In the above method, since ink is ejected from a very small hole with a pressure for deforming a part of the ink chamber wall, which pressure is set to about the atmospheric pressure, there does not occur any leakage of ink even when the small hole faces downward, but there easily arises the problem that the small hole becomes clogged due to drying of a solvent contained in the ink.
As an example of a method for preventing such clogging of the ink, there is a known method wherein fine holes are not provided, but ink ejecting electrodes are arranged on an open surface and a voltage is applied between any of the ink ejecting electrodes and a recording medium, allowing an ink droplet to be ejected with an electrostatic force.
In this method, since the amount of ink to be ejected is determined according to the width of the pulse which is applied, it is possible to change the diameter of each printing dot, and, hence, an image of high definition can be printed.
As an example of the use of this method, reference is made to the method disclosed in Japanese Patent Laid-Open No. Hei 7-502218, in which an ink with a colorant dispersed at a low concentration in a solvent is fed to a surface of an ink ejecting electrode and a voltage is applied to the ink ejecting electrode to create an electric field, allowing the colorant having an electric charge to be aggregated near the ink ejecting electrode and allowing the ink to be ejected from a slit-like open surface onto a recording medium. Further, an improvement of this method is disclosed in Japanese Patent Laid-Open No. Hei 7-76506.
Ink jet printers are based on either a printing method (using a scan type head) wherein a printing head is mounted on a carriage and ink is ejected for printing while the carriage is reciprocated in directions perpendicular to a recording medium conveying direction, or a printing method (using a line type head) wherein a printing head having the same width as the width of a recording medium is fixed in position and ink is ejected for printing onto the recording medium being conveyed. In case of home or office printers, the scan type head is desirable because a small size is required before everything else, while in the case of printers for business use, the line type head is preferred because a high-speed printing is required.
In both methods disclosed in the above Japanese Patent Laid-Open Nos. 7-502218 and 9-76506, the agglomeration of ink is prevented by circulation of ink even if a slight amount of the solvent vaporizes during printing, for which reason the occurrence of ink clogging is presumed to be unlikely. However, near the ink ejecting electrodes which open into the atmosphere, the pressure and gravity are apt to cause ink to leak to the exterior due to circulation of the ink, so that how such leakage of ink can be prevented is a subject to be considered in case of circulating the ink. A simple solution is to direct the open surface upward relative to a horizontal level.
In the configuration disclosed in the above Japanese Patent Laid-Open No. Hei 7-502218, ink is fed to a row of ink ejecting electrodes arranged horizontally, and an open surface is set horizontally. In the printer of this configuration, since the amount of ink to be ejected is controlled by the pulse width of the applied voltage, it is a very important to keep the spacing between the ink ejecting electrodes and the recording medium constant, thereby to keep electric field exerted between the two constant. In the case where the ink ejecting direction is the horizontal direction, and for keeping the spacing between the ink ejecting electrodes and the recording medium constant, it is necessary that the recording medium be conveyed while being brought into close contact with a common electrode at the position opposed to the ink ejecting electrodes.
As means for conveying the recording medium, there are a method in which the recording medium is wound around a roll and a method which uses a component capable of keeping the recording medium in close contact with the common electrode. However, both methods require the provision of separate components, so there arises the problem that the printers become complicated in structure.
The present invention has been accomplished for attaining or solving at least one of the above-mentioned problems.
To be more specific, it is an object of the present invention to keep he spacing between ink ejecting electrodes and a recording medium constant easily during conveyance of the recording medium, thereby to stabilize an electric field between the electrodes and the recording medium and hence stabilize the amount of ink to be ejected.
It is another object of the present invention to prevent the leakage of ink by feeding the ink at a stable pressure to ink ejecting electrodes and thereby prevent the portions around the electrodes from being clogged with the ink.
It is a further object of the present invention to accommodate a printing head composed of an ink tank and an ink circulating path compactly in a printer, to thereby reduce the size of the printer.
According to the present invention there is provided an ink jet printer comprising a recording medium conveying portion for conveying a recording medium, a head portion which causes ink to be ejected onto the recording medium conveyed by the conveying portion while allowing a colorant component contained in the ink to be aggregated on ink ejecting electrodes under the action of an electrostatic field, and a fixing portion for fixing an image printed by the head portion, the head portion causing the ink to be ejected below the horizontal level.
According to an embodiment of the head portion, the head portion comprises an ink chamber for the storage therein of ink with a colorant dispersed in a solvent, an ink circulating portion for circulating the ink from the ink chamber, and an ink ejecting portion having ink ejecting electrodes for ejecting the ink to the recording medium, with the ink being fed to the ink ejecting electrodes from the ink circulating portion, the ink ejecting electrodes ejecting the ink toward counter electrodes opposed to ink ejecting electrodes under the action of an electrostatic field, the ink chamber being provided above the ink circulating portion, and the ink ejecting portion being provided below the ink circulating portion, thereby permitting the ink to be fed from the ink chamber to the ink ejecting electrodes under the action of gravity.
Preferably, the ink circulating portion is provided with an ink sump portion for storing an appropriate amount of ink from the ink chamber, an ink flow control chamber for controlling the flow rate of ink and feeding the ink to the ink ejecting portion, and a pump portion for circulating the ink.
Preferably, the pump portion is provided with means for feeding ink to the ink ejecting electrodes and means for recovering the ink from the electrodes, and the ink sump portion is provided with colorant concentration detecting means for detecting the concentration of circulating ink.
Preferably, an uneven structure is provided on a wall surface by subjecting to labyrinth processing of an opening portion of the ink ejecting portion on the side opposite to the ink ejecting electrodes to increase the surface area.
Preferably, an ink-repellent substance is applied to tip end portions of the ink ejecting electrodes arranged in the opening portion of the ink ejecting portion and also to an ink flow path located near the opening portion.